Major floristic trends in Patagonia during the late Paleogene-early Neogene: evidence from spores and pollen grains

BARREDA, V.; PALAZZESI, L.

Bonn

Congreso; 12th International Palynological Congress; 2008

Patagonian vegetation faced major changes in composition, structure and distribution during the late Paleogene-early Neogene. The fragmentation of temperate rainforests and the expansion of sclerophyllous communities in eastern Patagonia are the most important turnovers. Fossil pollen records suggest that Paleogene landscapes were dominated by a Gondwanic group of families comprised mainly by Nothofagaceae, Podocarpaceae and Araucariaceae, along with Gunneraceae, Cunoniaceae and Proteaceae. By the Early Miocene rainforest taxa began to decline, some species became extinct (Lagarostrobos, Microcachrys, Nothofagus subgenus Brassospora) and others were confined to moister enclaves (Araucariaceae, Nothofagus subgenus Fuscospora). A new vegetation type composed by drought-tolerant plants progressively spread during the Neogene; Asteraceae (Nassauviinae, Chuquiraga), Calyceraceae (Boopis), Fabaceae (Caesalpinea, Anadenanthera, Prosopis), Anacardiaceae (Schinus), Poaceae, Chenopodiaceae, Convolvulaceae (Cressa) were among the most characteristic families. Some of them may have evolved from humid or sub humid ancestors as an adaptation to the new conditions (Asteraceae Barnadesioideae); others may have dispersed into eastern Patagonia from Neotropical bordering regions (Fabaceae Caesalpinoideae). The fragmentation of the rainforest communities was probably related to major global (cooling trend) and regional (Andean uplift) events, which would have led to a progressive desertification of the eastern Andean region. Such conditions allowed low trees and shrubs of Celtidaceae (Celtis), Anacardicaceae (Schinus) and Fabaceae (Prosopis, Prosopidastrum) to grow and expand by the Late Miocene. Many of these sclerophyllous taxa are today restricted to the seasonally dry forests of the Chaco Domain and might have derived from these Miocene ancestors.